1. Field of the Invention
The invention relates to laser wavelength control, and more particularly, wavelength feedback control of a tunable laser based on the Young's interferometer configuration.
2. Description of the Related Art
A laser wavelength meter, or wavemeter, is an instrument that directly measures the wavelength of light emitted by a laser. In the past, wavemeters have been commonly used in spectroscopic studies to monitor the wavelength of a tunable laser that excites some atomic or molecular transition. More recently, wavemeters are used to measure the wavelength of telecom laser sources in Wavelength Division Multiplexing (WDM) networks in order to ensure that the source is properly tuned to its channel in the ITU grid.
The accuracy required of a wavemeter depends on the application: Linear spectroscopy and telecom applications typically require accuracy of about 10−6 while non-linear spectroscopy may require a more demanding 10−7 to 10−8. Commercially available wavemeters offer accuracy ranging from a few parts in 10−5 to a few parts in 10−7. 
With few exceptions, wavemeters are based on some type of interferometer. The most common class of wavemeter is a form of Michelson interferometer in which the incident laser beam under test is divided by a beam splitter, sent down two different paths to moving retroreflectors that return the beams with smoothly varying optical path difference, and then recombined by the beam splitter to form fringes. See for example U.S. Pat. No. 4,319,843 (Gornall). The fringe intensity, which oscillates as the optical path difference changes, is detected, and the oscillations counted over some time interval. A reference laser beam, of precisely known wavelength, is injected into the same interferometer in parallel with the laser beam under test, and the oscillations of the reference laser's fringes are counted over the same interval. The ratio of the two fringe counts is equal to the inverse ratio of the wavelengths, and since one wavelength is known, the unknown wavelength is easily calculated.
Michelson wavemeters are conceptually simple, straightforward to construct, and capable of high accuracy. However, they require a reference laser as well as a translation stage to move the retroreflectors over relatively long distances at constant velocity, and are therefore not very compact or robust, and do not provide very rapid updates. In addition, since they must count every fringe during a scan distance of the order of a centimeter, they are only applicable to relatively narrow bandwidth cw lasers.
U.S. Pat. No. 4,173,442 (Snyder) discloses a wavemeter based on the Fizeau interferometer. This interferometer consists of a glass plate with a slight wedge that is illuminated by a collimated laser beam. The reflections from the first and second surfaces of the glass plate generate collimated beams that propagate in slightly different directions. Therefore, the two beams produce a pattern of straight, uniformly spaced, sinusoidal fringes over their overlap region. Snyder teaches that this fringe pattern, if recorded by a linear photodiode array and digitized, could be analyzed to determine the wavelength of the laser beam. Unlike the Michelson wavemeters, the Fizeau wavemeter is calibrated during manufacture, and does not require a reference laser. Because the photodiode array records the instantaneous fringe pattern, both cw and pulsed lasers can be measured. Moreover, since it has no moving parts, the Fizeau wavemeter is inherently more robust and can provide a higher measurement update rate than the Michelson wavemeter.
Although the Fizeau wavemeter offers advantages over the Michelson wavemeter, it requires precise opto-mechanical alignment, and it was found to suffer from systematic errors related to chromatic and other aberrations, and from thermo-mechanical instability. Some of these problems are addressed by U.S. Pat. No. 5,420,687 (Kachanov) and U.S. Pat. No. 5,543,916 (Kachanov), who simplified the optical system by eliminating the collimating mirror, and replacing the wedge with a glass plate with parallel surfaces. The Kachanov design produces a fringe pattern similar to the Fizeau, that can be analyzed in the same way. Although the Kachanov design is simpler than the Snyder design, the alignment requirements are similar and in practice the performance is not improved. In addition, it proved technically difficult to reduce the package size of either the Fizeau wavemeter or the Kachanov wavemeter much below the size of other wavemeters of comparable accuracy.
As a conventional tunable laser may require a high quality wavelength control system to meet required standards of various laser applications, the inaccuracy and complexity of an existing wavemeter may translate into the inaccuracy in the laser output and/or complexity of the wavelength control system. Thus, there is a strong need for a control system to monitor and control the wavelength of tunable laser light based on a wavemeter that is simple, yet can provide enhanced accuracy in wavelength measurements.